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Vintage Television and VideoVintage television and video equipment, programmes, VCRs etc.

This deceptively simple power supply is posing questions. According to the circuit diagram the PT650 was also designed to operate on DC mains supplies of 210 to 240 volts and yet there no alteration to the mains dropper adjustments. The same ohmic values apply for either AC or DC mains.

"I can't see why you would need a different tap for DC vs the same RMS AC.
Not for the heater chain anyway."

Certainly as for as the heaters are concerned it shouldn't matter if it is AC or DC mains. I've always been aware of the lower ohmic values of the HT sections of the mains dropper but this the first time on any forum that there has been any discussion about the subject.

The trader sheet for the Ultra V17-72 (VP17-72, etc) which is essentially the same chassis (sheet 1454) does give different dropper settings for AC and DC mains. The heater section (Blue and Green leads) is set the same way for DC mains of a particular voltage and AC mains of the same RMS voltage, but the HT section (Red and Yellow leads) is not (and I am not yet convinced there isn't a typo in the table...)

From the table given in Trader sheet 1270 for the Pilot PT650, I have worked out all the dropper values for both AC and DC for different mains voltages. (I should probably get out more )
As Tony says, there is no difference in the heater dropper values between AC or DC mains. The HT is a different matter however.

The above values are in ohms and ignore the fractional values.
So at 210V, the DC resistance is 26 Ohms less than the AC value, but at 240V it is 12 Ohms more!!
Is that a Typo or is there something strange going on?
I am not sure what to make of all that!
Cheers
Nick

One interesting thing about this sort of power supply circuit is that while running from AC, the HT voltage one ended up with becomes very dependent on the exact value of the first filter capacitor C34 (100uF) because of the substantial ripple voltage there sagging down between the half wave peaks.

Whereas the filter caps after the choke will have much less effect on the average DC values.

That 100uF value was probably specifically chosen so that if DC is applied (substituted for AC) where the DC matches the RMS voltage of the AC, one ends up with the approximate same voltage after the filter choke in both the AC & DC case.

Given the lousy tolerance of electros, it would be well worth checking that value if not already done. There would be a significant shift in HT voltage if it were say 50uF or 150uF.

In the case of the heater chain it is much easier, all they had to do was to ensure that the rms AC voltage matches the DC value, which for heating effects it always does anyway. But the HT voltage side of things with a half wave rectifier & filter system is a trickier balance so that you get the same result on AC or DC.

The receiver for this experiment is the GEC BT5144 which is the subject of another topic in this forum.
The HT reservoir capacitor in the set has been disconnected and replaced with a separate 100mfd capacitor which has a 1 ohm resistor wired in series with the capacitor's negative chassis connection.
The lower scope waveform shows the voltage developed across the resistor. The scope reading shows slightly more than one volt across the resistor thus indicating more than one amp flows into the capacitor during the positive mains cycle. There is also a steady negative voltage reading of 0.25V which indicates the current is actually flowing out the capacitor during the negative half cycles of the mains waveform. That 0.25volt developed across the 1 ohm resistor almost equates to the HT current demands of the receiver, 250mA.

That's a very interesting experiment, David and most informative! I'd never given ripple current much thought- up until now I suppose during the negative half cycle, the entire HT current has to be supplied from the resevoir and smoothing caps. In this case, then, the ripple is roughly 4 times the HT current so during these peaks the power dissipated by the dropper will be 1A ripple + .25A HT current. 1.25 squared * 100 ohms (dropper resistance) =156 Watts!!
Looking at the scope trace, it's only doing this for about 1/5 of a cycle, the rest of the time the HT current is being supplied by the capacitor, so no power will be lost in the dropper. Whether we can divide the 156W peak by 5 as an approximation of the average power, I don't know. Even so, that would still give 31W. Mind you, that is what my 100 ohm dropper seems to be trying to dissipate!
Out of pure interest, if we were to use a full wave bridge, the ripple would be significantly reduced. Would this in turn reduce the power dissipated by the dropper?
Many thanks for doing the experiment
All the best
Nick

"Out of pure interest, if we were to use a full wave bridge, the ripple would be significantly reduced. Would this in turn reduce the power dissipated by the dropper?"

Hi Nick,
During the course of the topic about the Baird M676 mono TV I mentioned that the special Baird schools set employed an isolation transformer and a bridge rectifier to supply the standard "640" series chassis."A mains isolation transformer was a safety requirement demanded by certain education authorities so this was fitted in the special receiver. Note the BY100 silicon full wave rectifiers which are used to supply the unmodified 640 chassis." While the scope and test components are hooked up to the GEC BT5144 there is no reason why a bridge rectifier shouldn't be tried out in the mains input supply to the receiver.
The results might prove to be interesting. It is assumed that the chassis of the Baird schools set was not modified after the addition of the bridge rectifier.

The results are interesting. The upper waveform is the 100Hz voltage from the bridge rectifier. The lower waveform indicates that at 1amp the charging current is only slightly less than readings given in the previous experiment with the 50hz half cycles. (1.2amps) The discharge current is up to 0.3amp and the reason for that is because the HT voltage has risen from 220 volts to 252V.
Likewise the voltage across the reservoir capacitor has risen from the previous reading of 240 volts to 275V.

The GEC BT5144 on test. The RM4 metal rectifier was replaced with a BY127 silicon diode. A 30 ohm series resistor has been added to compensate for the lower forward voltage of the silicon diode compared with old type selenium rectifier. The 30 ohm resistor is a convenient test point to measure the HT current. When the receiver was supplied from the bridge rectifier the voltage drop measured 6.5volts. After removing the bridge rectifier the voltage drop across the 30ohm resistor is 7.9 volts.
I might add that when the bridge rectifier was in the mains input the HT rose by 30volts and to return the HT supply to the correct value the variac had to be turned down to 205 volts.
The test receiver is now supplied direct from the mains which measures here as 242volts. The HT voltage has risen slightly and is nearer 230 volts, also now that the power supply only receives the positive half cycles at the 20mS rate the reservoir capacitor current has risen to 2amps during those positive half cycles. The discharge current during non conduction period of the rectifier is up slightly to 275mA.
Although the Pilot PT650 employs a 110 degree wide angle deflection CRT the total HT current demand will be thanks to the improved efficiency of the line timebase circuits very similar to the old GEC set I've used for the experiments.

Many thanks for using the GEC as a test bed for these experiments. If I'm reading the results correctly, it would seem that with full mains applied, the ripple current is 2A with the single diode and 1.2A with the bridge. Theoretically, this would result in less heating of the dropper but as the HT rises with the bridge, a bigger resistor would be needed to bring the HT down to the correct value. Thus resulting in more heat being dissipated!
So all in all, there is probably not much difference in the heat dissipated by the dropper between full or half wave rectification to achieve the same HT voltage. Interesting experiment though!

You mentioned the more efficient line timebase in the Pilot for the 110 degree tube. I had been puzzling over the (to me) slightly strange circuit where the voltage for the anode of the efficiency diode does not come directly from the HT rail. Instead it is fed via a choke and the secondary winding of the LOPT. This winding (which drives the scan coils) is connected to the main primary via a capacitor.
I happened to read the 'developments in tv receivers' section in the '59/60 red Newnes book. This details a 'de-saturated' line output transformer whereby the DC current for the line o/p valve is fed in the opposite direction through the secondary winding.
This neutralises the DC component in the transformer and results in greater efficiency. It also reduces the intensity of the line whistle.
All very interesting I thought and not something I was aware of!
True enough, the line whistle from the Pilot is much less piercing than the GEC 2000!
All the best
Nick

The Pilot PT650 has a vey low valve count. Having only eleven valves in the heater chain means that the mains dropper is dissipating 31 watts of heat when the receiver is supplied from 240 volts.
The total voltage across the heater chain is 135V so 105 volts has to be dissipated in the form of heat, that's not good especially one you consider the frame timebase components are very close to the mains dropper resistor.

If we check out the number of valves employed in the BRC 1400 we find that this receiver also has a low valve count and the total heater chain voltage adds up to 140V, only five more than the Pilot. However, the 1400 employs a silicon diode in series with the heater chain so that it is only the negative cycles of the mains supply that provide the power. The negative going heater supply also provides the grid bias for the frame output valve.
Another good feature of having the valve heaters supplied from the negative half cycles is that it almost balances the demands of the HT supply.
The 1400 has a 128ohm resistor in series with the heater chain so if we assume a current of 0.3amp the heat dissipated by this resistor will be just over eleven watts.

Good evening,
I thought it was high time I did some more work on this project: With the cold winter nights its very handy to have a TV that produces so much waste heat
I put it on the bench and powered it up to refresh my memory as to where I had got to with it. It soon produced a geometrically very good picture but with awful definition due to the IF stages requiring a full alignment.
It wasn't long before the smell of over cooked mains droppers became overpowering so that decided my plan of attack!

The dropper has to dissipate something in the region of 45W between the heaters and HT sections which is a huge amount of heat. The original was a pretty large thing but was totally knackered and I suspect it was not really up to the job when the set was new.
With some careful juggling of values, I have now used two droppers- one the exact physical size of the original and a second slightly shorter one.
This works very well and gives the correct HT and heater voltages. The HT section had to be higher to compensate for the use of a silicon diode in place of the metal rectifier. So the heat dissipation is now spread over a much greater length of dropper.
There is sufficient space to mount the second dropper below the first one so I think this may well have to become the final solution.
Anyway, it allows me to run the set for longer periods without the solder melting
I can now turn my attention to the low EHT issue. It only reads 10KV with a normal brightness picture displayed rather than 16KV so that will be the focus for tomorrow night.
All the best
Nick

Good evening,
Well, despite my best efforts, the EHT obstinately remains at 10KV. I tried a new U26 rectifier, 30P4 LOP valve (and a PL36 & 30P19 as well).
All the other components were new anyway. The picture size & linearity are good and in fact, the picture is not too bad, it's just the EHT should be 16KV.
My conclusion, therefore, is that it's the LOPT.

We have recently been discussing the issue of the wax or pitch coatings reducing the 'Q' of the transformer and therefore the efficiency. Any major failings, such as shorted turns would have a more dramatic effect.
This LOPT has the primary windings on one 'limb' and the EHT winding on the other. The primary is wax impregnated but the outer layer is just stiff paper. The EHT winding has a thick layer of wax all over.

By way of experiment, I measured the boost and EHT volts and the temperature over 1 hour.
At the start, boost was 525V and EHT was 10.5KV
After 1 hr boost was 485V and EHT was 9KV
Temperature of the inner part of the primary near the core was 50C and the EHT winding was 40C. Although I would not be unduly worried about these temperatures, the wax was starting to bubble on the primary and was soft on the EHT winding. Also a good discharge was obtained to an insulated screwdriver held onto the wax so it's not doing a great job of insulating!
In the past, I have mounted small cooling fans near the LOPT to keep them running cool which works well so long as the EHT starts off OK.

So there are two courses of action open to me. Either fan cool it and live with low EHT, or experiment with removing the wax & re- coating it in the hope that things will improve.
Obviously, the second option is much more interesting, even though it might kill it completely!
So that's what I'm going to do, inspired by Argus's de- waxing methods and general insight.
Earlier today, I dropped a small piece of the wax coating into some White spirit to see how good a solvent it was. It's very slow, but it does soften it and with some agitation will dissolve it. I might consider placing the wax covered winding in a small oven at 60- 70C and letting the thick wax drip off before using the white spirit.
White spirit is pretty tame and hopefully won't affect the other component parts of the winding.
It then leaves the question of what to re- coat the windings in?

The LOPT from the GEC 2000, which is of later construction, uses a sort of plastic encapsulation over the EHT winding rather than wax. Despite the cracks in it, it gives full EHT which doesn't change over time.
So we will have to wait and see what this experiment will prove: it may all end in tears!!
All the best
Nick

Earlier in this thread, John W (HCS) suggested you could hook up any LOPT to the top caps of the LOP valve and the boost diode and it would work well enough to light the heater of the EHT rectifier. Ever the sceptic, I wasn't convinced it would work without scan coils, boost cap etc. connected as well, so I have been waiting for the chance to try it for myself!
To be fair, John said it probably wouldn't work that well with the circuit arrangement in the Pilot. That's because feedback for the line oscillator comes via a cap from the LOPT so the oscillator doesn't work in isolation from the LOPT.

I have the LOPT removed from my Marconiphone VT 161, which I thought had died and which I was going to get re wound at some point. When I was doing my 'ringing' experiments last year to see how good and bad transformers behaved, I was very surprised to find that this transformer, which I was sure was dead, gave a good 'ring' trace on the scope, indicating that it might well be OK.
So I hooked it up to the Pilot to see what would happen!

The Pilot uses a 'de- saturated' LOPT (see post 113) whereby the HT current is passed through the winding that drives the scan coils via a width choke. This neutralises the DC component in the primary winding and results in greater efficiency.(as far as I understand it)
I had to modify this arrangement to make it more similar to the VT161, taking the HT direct to the boost diode cathode.
Switching on, very little happened (I'd already taken feedback for the line osc via a cap from the LOPT). I then added a boost cap in the same configuration as the VT161 circuit and the thing became much more lively.
I eventually had to replicate the circuit of the VT161 before the transformer would oscillate properly at the correct frequency. When it did, the EHT rectifier heater lit up nicely.

However, it point blank refused to drive the scan coils- I suppose the impedance was very different. Hardly surprising as the Pilot is 110 degree narrow neck tube and the VT161 is 90 degree fat neck tube.
I really wanted to measure the EHT though so I had to connect up to the CRT final anode so that the smoothing cap formed by the CRT was present. Obviously, I had no scanning capability so had to ensure that the CRT didn't produce an intense white dot!!
With the brightness at minimum, I switched on. Everything was OK, the screen was completely blanked and the EHT measured 15KV!! So I am pretty confident that the VT161 transformer is viable. The EHT winding is covered in melted wax though....

The problem came when I thought what would happen when I turned the TV off. With no beam current, the EHT would stay charged and as soon as the voltages on the CRT electrodes collapsed, there was every chance of a major intense white dot on the screen!! I really hadn't thought that one through
I had visions of having to leave the set on for ever because it would kill the tube if I turned it off!
Eventually I decided that the best method was to let the CRT heaters cool so there would be no emission and hence no dot. Pulling an IF valve broke the heater chain and let everything come gently back down to earth with no drama
So my conclusion from all that is that my VT161 transformer is probably fine and that the one in the Pilot is not as good as it should be.
All in all, an interesting experiment, (for me anyway!) so thanks John for putting the idea in my mind
So for better or worse, I am going to remove the wax and re- insulate the Pilot transformer and see what happens!
All the best
Nick

Last edited by 1100 man; 10th Dec 2017 at 12:42 am.
Reason: not enough words in this post!!

Hi,
I took the plunge and removed the LOPT and stripped it down as far as possible. The core didn't want to come apart with very mild force, so I left it complete- it shouldn't get in the way of what I want to do.
There was a very thick coat of wax on the EHT winding so I decided some gentle heat to soften it would be helpful. I left it in a small table top oven at about 80 degrees for about half an hour, checking every 10 mins to see if the wax was soft. It got to a point where I could gently remove the bulk of it with a screwdriver.
During the course of two days, I left it soaking in a container of white spirit. This took a while to soften the wax but it did break it down in the end with a little help with a paint brush now and again.
It didn't seem to dissolve anything it shouldn't and the transformer is now pretty wax free. How much is impregnated within the windings I don't know. Whether they were just dipped originally or were pressure treated would determine that.
My current theory is that the problem was mainly due to leakage from the EHT winding to the surrounding air and also to the transformer core dragging the EHT down. A reasonable spark could be drawn from the wax coating anywhere on the winding. So if that is true, then just re- insulating it should help. Any wax within the windings won't do too much harm, if leakage was the only factor.

I searched for something suitable to re- cover the winding in. RS do an 'insulating varnish' which is specified for transformers, PCB's etc. It is rated at 60Kv/mm, so hopefully, several coats with a small brush should be adequate. It's reassuringly solvent based, carcinogenic and comes with a host of warnings on the tin so might be a good product!! I don't trust anything that doesn't contain dangerous chemicals I've certainly never had much luck with water based products!
This turned up in a huge box which I expected to contain a massive H&S data sheet, but not a word, only packing materials! When I ordered Araldite, it came with 36 pages (I kid you not) of Hazard information!

I will let the transformer dry out for a week or so before varnishing and then it's fingers crossed time
You can see how thick the wax coating was if you look at the metal dome where the top cap of the rectifier was connected- this was below the surface of the wax!
That's it for the moment,
All the best
Nick

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